/Engine/Source/Runtime/Slate/Public/Framework/Views/TreeFilterHandler.h
Gets the children for a node in the tree which pass the filter if filtering is enabled, or gets all children if the filter is not enabled. In order for filtering to work correctly this function must be used for the TreeView's OnGetChildren delegate.
void OnGetFilteredChildren ( ItemType Parent, [TArray](API\Runtime\Core\Containers\TArray)< ItemType > & FilteredChildren )